1. Field of the Invention
This invention relates to refrigeration and air conditioning and particularly relates to increasing the thermodynamic efficiency of vapor compression systems used for air conditioning of automotive passenger compartments. It especially relates to automatic control during bypass of liquid from the compressor and to combining of fluids having diverse temperatures.
2. Review of the Prior Art
The trend in automotive air conditioning systems is to replace the thermostatic expansion valve with a less-responsive liquid-control device, called an "expansion tube," which in essence is a short-bore capillary approaching an orifice. The primary reason for this substitution is that a thermostatic expansion valve is not completely dependable and is relatively expensive. However, the expansion tube is not responsive enough to prevent over-feeding under all operating conditions and is certainly much less responsive to variations in system operating conditions than is a thermostatic expansion valve. Therefore, at many operating conditions the expansion tub over-feeds the evaporator with liquid refrigerant.
To prevent this excess liquid from reaching the compressor in damaging quantities, a liquid trapping suction accumulator is more necessary than usual and is used, as is customary, to catch and meter the liquid return at a rate that will not damage the compressor. With this arrangement, all of the unevaporated liquid entering the accumulator must eventually pass through the compressor, either in the form of refrigerant vapor created by the transfer of heat from the engine compartment through the wall of the accumulator, or in the form of atomized liquid droplets that are metered into the suction vapor flow going to the compressor via the accumulator. The result of this process is that the mass flow through the compressor is greater than the mass flow required to provide the useful refrigerating effect in the evaporator. This additional mass flow rate requires the compressor to do additional work and therefore to consume energy beyond that required to provide the desired cooling capacity.
The performance of the expansion tube is based primarily on two factors. One is the pressure differential from inlet to outlet of the tube, so that as the pressure differential increases, the mass flow rate through the tube increases. The other factor is the condition of the high-pressure liquid entering the tube with respect to the amount of sub-cooling in the high-pressure liquid. In other words, the amount of sub-cooling is measured by how much the temperature of the liquid is reduced below its saturation temperature. The way that sub-cooling affects the flow rate through the expansion tube is that the greater the sub-cooling in the entering liquid, the greater is the flow rate through the expansion tube so that the two factors, the pressure differential and the sub-cooling, then combine to give a finite flow rate of liquid through the expansion tube.
The process by which sub-cooling affects the flow rate in the expansion tube is that a pressure gradient exists across the expansion tube and as the high-pressure sub-cooled liquid enters the expansion tube, the pressure of that liquid begins to follow the pressure gradient line and reduces to the point of saturation. At the point of saturation, any further reduction of pressure which does occur causes the formation of vapor from the saturated liquid which then causes the balance of the tube length from that point on to perform as though it were handling vapor rather than liquid, thus causing the tube to choke and therefore giving a throttling effect and reducing the mass flow rate through the tube. Now, the way the expansion tube displays the control characteristics in the refrigeration system that is unique to refrigeration closed systems is that the amount of sub-cooling in the entering liquid varies with evaporator loading because of refrigerant charge distribution in the system. For example, at low-load conditions, one tends to have more of the refrigerant charge in the evaporator and hence less in the condensor. Therefore, the liquid entering the expansion tube is not sub-cooled as much; this situation causes the "bubble void" (defining the point where vapor first forms) to move upstream and causes more of the tube length to operate under vapor-flow conditions, thereby causing an increased throttling effect to occure that reduces the total flow rate. Conversely, when the evaporator load is high, the evaporator tends to hold less of the refrigerant charge, moving more of it to the condensor where it is allowed to sub-cool more. The increased sub-cooling in the entering liquid causes the bubble point or point of saturation to move downstream in the expansion tube and then less of the tube length is operating under vapor flow conditions. Thus the throttling effect is relaxed.
Because the expansion tube performs in this manner in a normal refrigerating system, one always has two-phase flow exiting the expansion tube. Because two-phase flow is difficult to predict, especially in transient conditions such as those existing in nozzled exits, it is therefore very difficult to predict what the induced rate of flow through the side connection of the ejector will be with respect to the primary flow rate through the expansion tube. A range of induction ratios does in fact exist, and the only way to establish what that range is absolutely is by measurement.
The inside bore diameter of the expansion tube can varied from 0.030 inch to 0.125 inch, and the length of the expansion tube can be varied from one inch to four inches. A designer normally tends to ascertain optimum diameter and length for a specific fluid and refrigeration system and thereafter remain as close as possible thereto.
United States patents in which the motive energy in the fluid is used to compress the fluid within the refrigeration cycle include the following: U.S. Pat. Nos. 1,922,712; 1,972,704, 1,993,300; 2,088,609; and 3,670,519. Other United States patents which relate to controlling or responding to the condition of the lubricant are the following: U.S. Pat. Nos. 2,975,613; 3,379,030; 3,777,509; and 3,938,349.
These patents in combination indicate that the high pressure fluid can be used to entrain another fluid and to inject it into the evaporator and further show that the art has been aware of the need for circulating the oil to the compressor. However, neither singly or in combination has the prior art taught that liquid in the liquid trapping suction accumulator could be fed to the evaporator without a separate pump or that the liquid and/or vapor in the accumulator could be used for sub-cooling the fluid entering the evaporator while being warmed for feeding to the compressor.